Lamellar gels

Eccleston GM, Behan-Martin MK, Jones GR, Towns-Andrews E. Synchrotron X-ray investigations into the lamellar gel phase formed in pharmaceutical creams prepared with cetrimide and fatty alcohols. Int J Pharm 2000 203 127-139. 

Membrane lipids are invariably polymorphic that is, they can exist in a variety of kinds of organized structures, especially when hydrated. The particular polymorphic form that predominates depends not only on the stmcture of the lipid molecule itself and on its degree of hydration, but also on such variables as temperature, pressure, ionic strength and pH (see References 11 and 12 and article Lipids, Phase Transitions of). However, under physiologically relevant conditions, most (but not all) membrane lipids exist in the lamellar or bilayer phase, usually in the lamellar liquid-crystalline phase but sometimes in the lamellar gel phase. It is not surprising, therefore, that the lamellar gel-to-liquid-crystalline or chain-melting phase transition has been the most intensively studied lipid phase transition... 

The lamellar gel-lamellar liquid-crystalline (L - L ) phase transition, frequently also referred to as (chain-)melting, order-disorder, solid-fluid, or main transition, is the major energetic event in the lipid bilayers and takes place with a large enthalpy change. It is associated with rotameric disordering of the hydrocarbon chains, increased headgroup hydration, and increased... 

At temperatures below the main transition, a basic equilibrium stracture is the subgel (crystalline) Lc phase. Its formation usually requires prolonged low-temperature incubation. In addition to the Lc phase, many intermediate stable, metastable, and transient lamellar gel structures are adopted by different lipid classes—with perpendicular or tilted chains with respect to the bilayer plane, with fully interdigitated, partially interdigitated, or noninterdigitated chains, rippled bilayers with various ripple periods, and so forth. (Fig. 1). Several polymorphic phase transitions between these structures have been reported. Well-known examples of polymorphic transitions are the subtransition (Lc- L ) and the pretransition (Lp/- Fp/) in phosphatidylcholines (33). Recently, a polymorphic transition that included rapid, reversible transformation of the usual gel phase into a metastable, more ordered gel phase with orthorhombic hydrocarbon chain-packing (so-called Y-transition) was reported to represent a common pathway of the bilayer transformation into a subgel (crystalline) Lc phase (62). 

Highly ordered lamellar gel microstructures are formed by certain surfactants and mixtures of a surfactant and long-chain fatty alcohols in water. Using small angle X-ray scattering (SAXS), an ordered lamellar stack lattice model was proposed for the gel formed by 10% w/w cetostearyl alcohol containing 0.5% cetri-mide surfactant. In contrast, the microstructure of a Brij 96 gel depends on the surfactants concentration. A hexagonal liquid-crystalline gel structure was... 

Fig. 4. Schematic drawing of lipid-water mesophases (Lc, lamellar crystalline Lps Pp., lamellar gel L , lamellar liquid-crystalline Qn, Qn°, Qn , inverse bicontinuous cubics Hu, inverse hexagonal). The cubic phases are represented by the G, D, and P minimal surfaces, which locate the midplanes of fluid hpid bilayers.

Fig. 9. Temperature-composition isobaric phase diagram for the fully hydrated dipal-mitoylphosphatidylcholine/dipalmitoyl-phosphatidylethanolamine system constructed using the temperature gradient method. The notation used is that of Luzzati [8] and is as follows Lc, lamellar crystalline (also referred to as the subgel phase) L f, lamellar gel phase with hydrocarbon chains tilted with respect to the bilayer normal P)el, ripple phase L. lamellar liquid crystal phase. Insert bold line in graph as indicated...

Lamellar gels are also formed by soaps under appropriate conditions of concentration and temperature. These may be regarded as essentially micellar systems [Figure 11.2(d)] in which at high concentrations the lamellar micelles extend over considerable distances and interlock to form a continuous gel network. 

Battaglia G, Ryan AJ (2005) The evolution of vesicles from bulk lamellar gels. Nat Mater 4 869-876... 

Owing to the SC composition, its lipid phase behavior is different from that of other biological membranes. The hydrocarbon chains are arranged into regions of crystalline, lamellar gel, and lamellar LC phases, thereby creating various domains within the lipid bilayers [84]. Intrinsic and extrinsic proteins, such as enzymes, may also affect the lamellar stmcture of the SC. Water is a vital component of the structure, acting as a plasticizer to prevent cracking of the SC. 

M. Caflfey, Kinetics and mechanism of the lamellar gel lamellar liquid-crystal and lamellar inverted hexagonal phase-transition in phosphatidylethanolamine a real x-ray diffraction... 

Figure 3.4 The temperature-concentration phase diagrams of BPS-m (m=5,10,20, and 30) with BmimPFj. The phase abbreviations are as follows micellar phase (1 ), discontinuous cubic liquid crystal phase (1 ), hexagonal liquid crystal phase (H ), lamellar liquid crystal phase (L ), lamellar gel phase (L ), reverse micellar phase (L ), ionic liquid phase (IL), and two-phase separation (II). The chemical structure of j0-sitosterol ethoxylates as a typical example of BPS-m is also shown in this figure. Reproduced from Sakai et al. [37] with permission from Japan Oil Chemists Society.

LP = liquid lipophilic dispersed phase LN = lamellar gel network (continuous gel phase) CA = cavity. 

Figure 16.10. Typical scattering spectrum of a lamellar gel mesophase as a function of temperature, showing gradual melting, with the formation of an intermediate (hexagonal) phase at ca. IIO C (courtesy of R. Corkery)...

McGrath, K. and Kidman, M., Spiral textures in lyotropic liquid crystals First order transition between normal hexagonal and lamellar gel phases, J. Phys. II (France), 3, 903-926 (1993). 

An acid soap of 2 1 complex ratio was discovered recently between triethanolamine stearate and stearic acid (13). In water the acid soap forms a lamellar liquid crystal phase at high temperatures, above 60°C, and transforms to a lamellar gel phase on cooling. The gel phase, however, is not stable at ambient temperature due to the occurrence of the hydrolysis reaction which converts the soap back to stearic acid which precipitates in the triethanolamine aqueous solution. A polymorphism of C, E, and possibly A forms of stearic acid crystals were found resulting from the hydrolysis reaction (8). 

The decrease in the melting point and the H (heat of reaction or transition, or enthalpy of reaction) both suggest a lower degree of hindrance for the phase transition step this in turn may relate to break up of the structure of the lamellar gel-phase. 

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